1. Field of the Invention
The present invention relates to a transmission/reception apparatus and transmission/reception method that carries out handover in a communication system according to a TDMA (Time Division Multiple Access)-TDD (Time Division Duplex) system, and more particularly, to a transmission/reception apparatus and transmission/reception method that carries out handover in a communication system in which a plurality of user signals are multiplexed on time slots according to a CDMA (Code Division Multiple Access) system or OFDM (Orthogonal Frequency Division Multiplexing) system, etc.
2. Description of the Related Art
As multiple access technologies for radio communications such as car phones and cellular phones, a TDMA system, CDMA system and OFDM system, etc. are used. The TDMA system is a system in which users communicate with each other by dividing a time. The CDMA system is a system in which a plurality of users share a same frequency or same time and communicate with each other by separating each user's signal using an orthogonal spreading code.
Furthermore, as a system of multiplexing downlink (channel through which a base station transmits a signal to a mobile station) and uplink (a channel through which a mobile station transmits a signal to a base station), a TDD system and FDD (Frequency Division Duplex) system are available. The TDD system is a system of multiplexing the downlink and uplink in a same frequency band. The FDD system is a system of multiplexing the downlink and uplink in different frequencies on a same time.
In the TDD system in particular, a same frequency is assigned to transmission and reception, that is, a same frequency is assigned to the downlink and uplink, and therefore a frequency correlation of fading fluctuations between a transmission signal and reception signal in a base station and mobile station is 1. Furthermore, in the case where the cycle of switching between the uplink and downlink is sufficiently short, a time correlation between the downlink and uplink of the propagation condition such as fading fluctuations increases.
In a cellular system, handover is generally carried out. According to FIG. 1, when mobile station 11 moves, for example, from the area of base station (A) 12 to the area of base station (B) 13, handover is a technology for switching the traffic channel handled by base station (A) 12 to the base station (B) 13. There are two kinds of method for carrying out handover; soft handover and hard handover.
Soft handover is mainly applied to a CDMA system and is a method for switching traffic channels as described below. That is, when handover takes place, the traffic channel handled by base station (A) 12 and the traffic channel handled by base station (B) 13 are simultaneously maintained for mobile station 11 and then the traffic channel used by mobile station 11 is changed to the traffic channel handled by base station (B) 13 only. This soft handover can reduce a possibility that the traffic channel will be down during handover.
On the other hand, hard handover is mainly applied to a TDMA system or a communication using packet transmission and is a method for switching traffic channels as described below. That is, hard handover is a method that does not allow mobile station 11 to use the traffic channel handled by base station (A) 12 and the traffic channel handled by base station (B) 13 simultaneously, and changes from the traffic channel handled by base station (A) 12 to the traffic channel handled by base station (B) 13 at certain timing.
The operations of the base station and mobile station when these two kinds of handover are used in a TDMA-TDD based communication will be explained with reference to FIG. 2 and FIG. 3. Here, a case where a CDMA system is used as the method of multiplexing signals of a plurality of users in each time slot will be explained. As shown in FIG. 1, suppose mobile station 11 moves from the area of base station (A) 12 to the area of base station (B) 13.
First, the case where soft handover is used will be explained with reference to FIG. 2. FIG. 2A to FIG. 2E are schematic diagrams showing an example of state transition of time slot assignment in execution of soft handover in a conventional TDMA-TDD based communication.
As shown in FIG. 2A, before handover takes place, a downlink traffic channel used for transmission from base station (A) 12 to each mobile station (including mobile station 11) is assigned to time slot (hereinafter referred to as “TS”) 21 in a frame and an uplink traffic channel used for transmission from the above mobile station to base station (A) 12 is assigned to TS22 in the frame.
When handover starts, as shown in FIG. 2B, a downlink traffic channel used for transmission from base station (B) 13 to mobile station 11 is assigned to TS23 in the frame. At this time, mobile station 11 combines the signal received over the downlink from base station (A) 12 and the signal received over the downlink from base station (B) 13.
Then, as shown in FIG. 2C, an uplink traffic channel used for transmission from mobile station 11 to base station (B) 13 is assigned to TS24 in the frame. At this TS24, mobile station 11 transmits a signal to base station (B) 13 and base station (B) 13 receives the signal from mobile station 11.
When mobile station 11 enters the area of base station (B) 13 completely, as shown in FIG. 2D, mobile station 11 stops transmission to base station (A) 12, which was carried out at TS22. Then, as shown in FIG. 2E, base station (A) 12 stops transmission to mobile station 11, which was carried out at TS21. In this way, handover is completed.
Then, a case where hard handover is used will be explained with reference to FIG. 3. FIG. 3A and FIG. 3B are schematic diagrams showing an example of state transition of time slot assignment in execution of hard handover in the conventional TDMA-TDD based communication.
As shown in FIG. 3A, before handover takes place, the state of assignment of time slots in the frame is the same as that in the case of soft handover described above (FIG. 2A).
Then, triggered at certain timing while mobile station 11 is moving from the area of base station (A) 12 to the area of base station (B) 13, the state of time slots in the frame is changed from the state shown in FIG. 3A to the state shown in FIG. 3B.
That is, base station (A) 12 stops transmission to mobile station 11, which was carried out at TS31 and base station (B) 13 starts transmission to mobile station 11 at TS33. Moreover, mobile station 11 stops transmission to base station (A) 12, which was carried out at TS32 and starts transmission to base station (B) 13 at TS34. In this way, handover is completed.
However, the conventional TDMA-TDD based communication using handover has the following problems: First, when soft handover is used, each mobile station uses traffic channels handled by a plurality of base stations (a plurality of parties on the other end of communication) simultaneously during handover, and this means that transmission is carried out at time slots corresponding in number with the base stations above for each frame.
Because of this, current consumption at each mobile station increases and at the same time interference by a transmission signal of each mobile station with base stations other than the base stations above (base stations that are not communication targets) using the same time slots as those described above increases. To suppress influences of this interference, it is necessary to reduce the number of mobile stations to be multiplexed on the time slots above. This results in a reduction of the system capacity.
On the other hand, when hard handover is used, unlike the case where soft handover is used, each mobile station performs transmission using only a traffic channel handled by one base station all the time, and therefore it is possible to equalize current consumption at each mobile station and interference with the other base stations above to current consumption and interference for any time (during a normal communication) other than a handover period.
However, in the case where a mobile station is near areas of a plurality of base stations, or more specifically, the mobile station is on the boundary between the area of base station (A) 12 and the area of base station (B) 13 in FIG. 1, the level of the reception signal from base station (A) 12 and the level of the reception signal from base station (B) 13 may be balanced and the level of the reception signal may be small. In such a case, switching of hard handover often takes place, which increases the possibility that calls made by the mobile station will be down.